Abstract

Juveniles of species in which size influences survival should be selected to optimize their growth rate. Although the defense of feeding territories is a common foraging tactic among stream-dwelling salmonids, the direct benefits of territoriality in terms of growth rate have seldom been demonstrated in these fish. 1 followed individually marked juvenile coho salmon, Oncorhynchus kisutch, for 3 months in field enclosures. Territorial fish grew faster than nonterritorial fish (floaters) confined to the same area. Territorial juveniles face a growth mortality trade-off when predators are present, since the movements associated with territorial defense may increase their risk of being detected by predators. When piscivorous birds (common mergansers, Mergus merganser) were present upstream, territorial coho rarely left their territories, but did decrease the average duration of their aggressive interactions and movements, and the total amount of time they devoted to territorial activities and to moving. Some fish also allowed intruders to approach more closely before intercepting them. There were, however, no effects attributable to the frequency at which fish were exposed to a predator.